Jetta cylinder misfires

DDCYR40

VOLKSWAGEN

I've been on here with my car for sometime, 2000 Jetta GL 2.0 Automatic, 90,000 miles. I had 8 codes all at once, some having to do with air/fuel ratio, running lean, oxygen sensor, and 3 different cylinder misfires. I changed the o2 sensor, went 1000 miles or more no problem at all. Today, came back on, scanned, P0300, P0301, P0303, which is random misfire, cylinder 1 misfire, and cylinder 3 misfire. Before I had the o2 sensor fixed, the car was running with the light on for nearly a year, not knowing what was wrong. Is is possible that the car running lean or previous problems with the o2 sensor could have caused the plugs to wear? Is there anything else I should look at for this? I have had the car since 65,000 miles and no real maintenance has been done, should plugs be done anyways? Could this be serious? The car isn't running bad, it idles slightly rough when starting. Let me know. Thanks a lot.